1. Field of the Invention
The present invention relates to a method and an apparatus for controlling the priority of communication packets which are applied to exchanges based on a connection-type communication method in which communication paths and communication channels are determined in advance on the basis of call information prior to commencing communication between terminals, such as packet exchanges and asynchronous-transmission-mode exchanges.
2. Description of the Related Art
As is known, in packet switching systems and asynchronous-transmission-mode switching systems, packet communication is conducted after communication paths and communication channels are determined in advance on the basis of call information prior to commencing communication between terminals.
In such switching systems based on a connection-type communication method, call priority values are set by connection processing with respect to calls from a plurality of terminals, and the control of instantaneousness or abandonment concerning subsequent packet communication is conducted on the basis of the call priority values.
For instance, a conventional packet switching system of this type is so arranged that a plurality of terminals are accommodated in one packet exchange, and this packet exchange is connected to an adjacent packet exchange via a trunk.
Here, a plurality of terminals are similarly accommodated in the adjacent exchange as well.
Now, it is assumed that a communication request has been made from an arbitrary terminal accommodated in an exchange to a terminal accommodated in an adjacent terminal.
In this case, a call message is delivered from the transmission-side terminal to a control unit in the subject exchange.
Here, the control unit secures a communication path, a communication channel, and the like between the transmission-side terminal and the reception-side terminal through control which will be described later, and delivers a call message to the adjacent exchange vial a call signal line in the trunk in accordance with path information for each preset destination.
With respect to the adjacent exchange as well, the control unit secures a communication path and a channel leading to the destination terminal, and delivers the aforementioned call message to this destination terminal.
Subsequently, in a case where packet communication is conducted between the two terminals, the transmission-side terminal transmits a packet to the memory of the subject exchange via a data line, and transmits the packet to, for instance, a trunk interface via a communication path set by the call message.
On the other hand, the adjacent exchange also continues to send packets from a trunk interface of its own to a destination terminal through the memory via the data line.
In the packet communication based on the connection-type communication method of this type, a call message of a format configuration such as the one shown in FIG. 15 is employed.
This call message comprises destination number information expressed by a destination exchange number and a destination terminal number; a call priority value (n bits) expressing a communication priority value between the terminals; a channel number captured for a terminal line and a trunk conforming with information on a path to a destination number of the packet transmitting side; and other additional information expressing communication attributes and the like of a terminal.
When this call message is received by exchanges within the packet switching system, their control units determine whether the call message is reception from a terminal or reception from a transit trunk.
Then, if it is reception from the terminal, a call priority value in the call message is written in a storage area for the captured channel value in the call message, the storage area belonging to a relevant terminal number area of a call priority table which will be described later.
On the other hand, if it is reception from a transit trunk, a call priority value in the call message is written in a storage area for the captured channel value in the call message, the storage area belonging to a relevant trunk number area in the call priority table.
Subsequently, a path where packets due to that call conforming with the information on a path to the destination number is written in a path information section of a packet path table, a vacant channel is captured from a trunk or terminal being presented as path information, and its channel value is stored in a channel information section of a packet table and a captured channel field in the call message. A call message is then transmitted to an exchange adjacent in terms of the path information.
If this processing is repeatedly executed from a beginning terminal to an ending terminal, the packet path and the priority of that call is set.
In addition, a conventional communication packet communicated by the connection-type communication method of this type has had a format configuration such as the one shown in FIG. 16.
This communication packet comprises an expression of a channel number captured at the time of call setting and an expression of communication information of a terminal, and communication is effected between a terminal and an exchange or between two exchanges on the basis of this format.
In the above-described packet switching system, the communication packet is subjected to so-called priority control concerning instantaneousness or abandonment at the time when the communication packet passes each exchange located on its communication path.
To realize this priority control, a memory is provided in the exchange, and a control unit therefor comprises a priority control unit, a buffer, a packet path table, a buffer busy-detecting unit, and so on.
Furthermore, the priority control unit among them comprises a packet disassembly unit, a call priority table, a comparator, a data bus switch, and the like.
Upon receipt of a reception packet via a packet reception bus, the priority control unit in the memory extracts a channel number in the packet by means of the packet disassembly unit, and inputs this channel number signal to the call priority table.
At that time, the reception packet itself is transferred to the buffer in the memory via the data bus switch.
In the call priority table, at the time of setting a call effected prior to that packet reception, a call priority value (n bits) is set for each terminal and trunk in correspondence with each channel.
In this state, if a channel number signal is inputted from the packet disassembly unit, the call priority value of the reception packet is retrieved on the basis of this channel number signal, and its result is notified to the comparator as a call priority signal.
The comparator makes a comparison between the call priority signal of the reception packet with a lowest priority signal in the buffer.
The aforementioned data bus switch interposed between the packet disassembly unit and the buffer is subjected to opening/closing control as a result of comparison by the comparator and as a result of detection by the busy detecting unit as to whether or not the buffer is busy.
As a specific operation, when the buffer is busy, if the result of comparison by the comparator is:
Call priority signal of the reception packet&lt;lowest priority signal in the buffer, then the data bus switch is held open, and the reception packet is abandoned without being stored in the buffer.
In contrast, when the buffer is busy, if the result of comparison by the comparator is:
Call priority signal of the reception packet.gtoreq.lowest priority signal in the buffer, then the data bus switch is closed, and the reception packet is stored in the buffer.
As a result of this processing, a reception packet whose call priority set at the time of call setting by connection processing is higher, among the reception packets, is preferentially stored in the buffer which is in the busy state.
The reception packets thus stored in the buffer are subsequently read out sequentially and are transmitted to the transmission data line, with the result that the packets subject to communication are communicated preferentially beginning with those whose priority values are higher.
In the priority control in this priority control unit, the call priority values for determining the priority of the communication packets have been set as fixed values by connection processing at the time of call setting, as described above.
Namely, in a conventional switching system of this type, the priority control of communication packets has been conducted on the basis of call priority values set as fixed values at the time of call setting.
Meanwhile, among the items of communication information between terminals subject to this priority control, information which is important and information which is not very important are generally mixed.
According to the above-described conventional priority control method, even in cases where communication information between terminals which has relatively high importance and that which does not are mixed, it has been unavoidable to effect communication during a period of call connection on the basis of the call priority value set as a fixed value at the time of call setting.
For this reason, with the conventional switching system of this type, even information which is not very important is compelled to undergo packet communication processing in accordance with a high priority, thereby hindering the effective utilization of the memory in the packet exchange and the trunk transmission zone.